You are probably seeing the time of flight delay from speaker to mic, plus any processing delay in the DSP, since the t=0 point is set by the loopback which isn't subject to those delays. You can allow for that in the "Timing offset" field on the Measure dialog. If you use the "Estimate IR...
I have fixed that for the next beta.
Just windowing the IR won't give you an IIR equivalent, you need to create a minimum phase version of it first. To work around the bug you can use the "Minimum phase version" Measurement action on the All SPL graph and export that IR, which can have a zero...
Are you using a timing reference output Greg? I think I have found some odd behaviour when changing sample rate from the Measure dialog if there is a timing ref output, try with "no timing reference" if you are currently using one.
There was a report of the file dialog not appearing on macOS last September, restarting the laptop seemed to fix it. More on that one here. I will see if I can reproduce the issues you are seeing with sample rate changes, though an initial check using the built-in audio behaved normally.
Those are the equivalent of selecting a device in the ASIO4All panel. The preferences drop downs show the inputs and outputs of the selected devices, usually with generic names like "IN 0". Unlike ASIO4All, the device name does not appear in the input or output name.
The window reference time (and t = 0 for that matter) are not restricted to sample boundaries, so I'm not sure there would be any benefit to window widths in samples. I'll add an extra decimal place for window widths below 100 ms though.
There are 1 minute and 10 minute Leq figures in the SPL logger, there isn't a 30 second figure.
Differences in the RTA figures are likely to be due to differing mic responses, assuming no special adjustments are being applied such as diffuse field corrections.
The laptop would be fine, the conversions will not make any difference. Test discs do not have the signal REW uses for measurement. You can save REW's sweep to a file and play that back if you prefer. There is a general guide here.